Plug-type closure for bottles, composed unitarily of synthetic resin material, have been provided heretofore as a substitute for conventional bottle closures composed of cork. These plug-type closures are themselves frequently referred to as "corks" and can comprise an outer circular-cross sectional relatively thin-walled outer member, e.g. a hollow cylinder, connected at its bottom by an end wall to a cylindrical core member disposed centrally of the plug.
The core can be provided with a preformed corkscrew-shaped cavity into which a corkscrew can be driven to facilitate withdrawal of the plug from the neck and mouth of the bottle. Radial webs can be formed unitarily with the core and the hollow cylinder to bridge them in angularly equally spaced relationship.
These synthetic-resin or plastic corks can be used to close wine bottles at the end of the bottling process and can be of the type described in French Pat. No. 1 106 988.
In this arrangement the internal cavity is formed as a cylindrical bore in the wall of which a groove of helical configuration is provided of screwthread configuration, i.e. constant pitch, constant radius and constant depth by turning methods, e.g. of the type well known in the production of female screwthreads in the metal.
Such expensive fabrication techniques must be used because for practical reasons, the complex-shaped internal cavity cannot be fabricated during injection molding of the plug. If injection molding techniques are used, the plug cannot be removed from the core-forming member of the mold or can only be removed by expensive or complex manipulations.
Furthermore, when efforts are made to use conventional corkscrews or like tools to withdraw the cork from the neck and mouth of the bottle, the shape of the corkscrew is frequently not compatible with the shape of the cavity or groove and the cork-pulling stress can be applied nonuniformly so that the corkscrew frequently pulls free and/or the cork is damaged.
Another bottle cork of the plastic type is described in French Pat. No. 1 127 460. Here the webs lie tangentially at the corkscrew-receiving member so that the disadvantages enumerated above with respect to handling and fabrication are present as well.
Still another plastic cork is described in German patent document No. DE-OS No. 16 07 901. In this construction in place of a helical groove, the cavity is formed with a multiplicity of semi-circular disks which are spaced apart in accordance with the pitch of the turns of the corkscrew so that diagonal disk edges can project between the turns and enable the cork to be withdrawn from the bottle.
Difficulties have been encountered with this system since the corkscrew does not engage the disks with sufficient precision to avoid damage upon the execution of a pull on the corkscrew. Furthermore the same problem creates difficulties in the threading of the corkscrew into the cork. In the fabrication of such a cork, moreover, it is frequently necessary to assemble the plug from two parts which are individually injection molded and then joined together. This fabrication technique is obviously expensive.